A heterogeneous network may include a high power node with one or more low power nodes co-existing with the high power node. For example, low power nodes form small cells such as pico cells, femto cells and relay cells while high power nodes form macro cells, which in general have a much larger cell coverage than the small cells.
In order to improve capacity and cell edge performance of the macro cells, low power nodes may be introduced within the macro cell to form the small cells. In some scenarios, the density of the small cells may be quite high. In this scenario, mobility and associated overhead could become a concern for a UE, especially for users with medium to high mobility. For example, user equipment (UE) travelling quickly may experience frequent handovers when moving across the small cells. Specifically, as the UE moves closer to a small cell, handover conditions indicate to the UE that the UE should handover to that small cell. However, when the small cell has a small coverage, fast changing radio conditions exist at the small cell edge and due to the frequent handovers, handover failure rates could increase, thereby impacting overall mobility performance.
Further, interference issues exist between the high power and low power cells. To remove interference, one deployment could be that the small cells use a different frequency layer from the macro cells. For example, the macro cells may use a 700 Mhz frequency band while small cells use a 3.5 Ghz frequency band. However this is merely an example. Such deployment can be referred to as an inter-site carrier aggregation (CA) based scheme. In accordance with this deployment, interference issues may be relieved at least between the macro cells and the small cells.
Other ways to mitigate interference is the use of almost blank subframes (ABS) where an interfering cell may transmit nothing or minimal information at lower transmission power to allow communication between an interfered with cell and its user equipments.